This invention pertains to a method for the removal of dense, nonaqueous-phase liquid contaminants from the subsurface by in-situ conversion of the dense liquids to light, nonaqueous-phase liquids.
The contamination of groundwater by nonaqueous-phase liquids (xe2x80x9cNAPLsxe2x80x9d) is a major environmental problem in the U.S. and worldwide. Common organic contaminants include trichloroethylene, tetrachloroethylene, trichloroethane, carbon tetrachloride, and gasoline. Generally, organic-phase contaminants are categorized as either light nonaqueous-phase liquids (xe2x80x9cLNAPLsxe2x80x9d) or dense nonaqueous-phase liquids (xe2x80x9cDNAPLsxe2x80x9d), depending on their density relative to that of water. For example, NAPLs such as petroleum hydrocarbons, which have densities lighter than water, are classified as LNAPLs, while chlorinated solvents, which have densities heavier than water, are classified as DNAPLs.
LNAPL contaminants typically migrate downward through unsaturated zones in the Earth""s subsurface and pool when they reach the water table. Once in the subsurface, an LNAPL source can either spread due to advection, be trapped in pores, or dissolve to form a downstream plume of contaminant in the aqueous phase. Light contaminants tend to remain at the capillary fringe, which benefits remediation because the contamination source can often be effectively targeted using techniques such as vapor extraction, vacuum extraction, and various pump-and-treat methods.
DNAPLs typically pose a more serious environmental risk than LNAPLs. DNAPL contaminants are one of the most common problems affecting groundwater quality. This type of contamination can result from degreasing operations, underground disposal, leaking underground storage tanks, etc.
DNAPLs typically migrate below the water table and into the Earth""s saturated zone due to their higher density relative to water. Once the bulk DNAPL is in the saturated zone, further transport will depend mainly on subsurface morphology, gravitational forces, capillary forces, and pressure gradients. Eventually, much of the dense contaminant will either pool on top of low-permeability strata, or be trapped by capillary forces in the form of very small ganglia (sizes on the order of the pore scale). Because of this behavior, remediation of DNAPL-contaminated sites is more challenging than for LNAPLs. More specifically, locating a DNAPL source zone and targeting it is more difficult because of the depth and the extent to which it may have moved. Furthermore, most methods that induce mobilization of DNAPL source zones involve increased risks because gravitational forces can cause the DNAPL to move deeper into the subsurface.
Currently, the primary method of recovering DNAPL contaminants is through remediation methods such as surfactant flooding, foam injections, pump-and-treat, etc. However, DNAPL remediation has significant problems. See National Research Council, Groundwater and Soil Cleanup, pp. 4-11 (National Academy Press, Washington D.C., 1999). For example, J. C. Fountain et al., xe2x80x9cControlled Field Test of Surfactant-enhanced Aquifer Remediation,xe2x80x9d) Ground Water, vol. 34(5), pp. 910-916 (1996) describes an alternative remediation process using surfactants. While surfactant-aided processes offer significant benefits over conventional pump-and-treat methods, they increase the risk of inadvertent subsurface mobilization.
G. J. Hirasaki, et al., xe2x80x9cSurfactant/foam Process for Aquifer Remediation,xe2x80x9d) pp. 471-480, proceedings of the SPE International Symposium on Oilfield Chemistry (Houston, Tex., Feb. 18-21, 1997) describes another remediation process using foam. Foam processes offer similar advantages to surfactant flooding, while simultaneously providing improved mobility control due to the high apparent-viscosity of foam. These processes can be selective in targeting DNAPL source zones within heterogeneous strata, because most foams tend to break upon contact with organic phases. However, foam-injection processes can require very large pressure gradients, and typically do not address density problems that occur during mobilization.
U.S. Pat. No. 6,210,955 discloses a process of in-situ remediation of contaminated soils comprising introducing a treating agent (e.g., nutrients, surfactants, oxidants and solvents, and bacterial cultures) into the contaminated soil and transporting the treating agent to an underground in-situ treatment zone using a foam-based fluid.
Another significant problem associated with remediation is inadvertent mobilization of DNAPL contaminants. See K. D. Pennell et al., xe2x80x9cInfluence of Viscous and Buoyant Forces on the Mobilization of Residual Tetrachloroethylene During Surfactant Flushing,xe2x80x9d) Environ. Sci. Technol., vol. 30, no. 4, pp. 1328-1335 (1996). Because of this risk, a number of investigators have proposed techniques that involve density modification.
K. Kostarelos et al., xe2x80x9cA new concept: the use of neutrally-buoyant microemulsions for DNAPL remediation,xe2x80x9d) J. Contaminant Hydrology, vol. 34, pp. 383-397 (1998) discloses formulations that promote neutrally-buoyant displacements by controlling the density of DNAPL-containing microemulsions.
C. T. Miller et al., xe2x80x9cRemediation of DNAPL-Contaminated Subsurface Systems Using Density-Motivated Mobilization,xe2x80x9d) Environ. Sci. Technol., vol. 34, no. 4, pp. 719-724 (2000) discloses modifying the aqueous-phase density via the use of concentrated NaI solutions. In upward-directed flow experiments, these high-density aqueous phase solutions were able to displace continuous-phase DNAPLs.
S. Lunn et al., xe2x80x9cManipulation of density and viscosity for the optimization of DNAPL recovery by alcohol flooding,xe2x80x9d) J. Contam. Hydrol., vol. 38, pp. 427-445 (1999a) discloses an alternative method of decreasing inadvertent mobilization using upward-directed alcohol floods as a means to control the mobilization.
Other alternative methods for decreasing inadvertent mobilization involve density modifications of both DNAPL and LNAPL phases. See S. Lunn et al., xe2x80x9cRisk Reduction During Chemical Flooding: Preconditioning DNAPL Density in Situ Prior to Recovery by Miscible Displacement,xe2x80x9d) Environ. Sci. Technol., vol. 33, pp. 1703-1708 (1999b); and E. Roeder et al., xe2x80x9cSwelling of DNAPL by Cosolvent Flooding to Allow its Removal as an LNAPL,xe2x80x9d) 333-345, proceedings of the ASCE Specialty Conference, Washington D.C. (Nov. 12-14, 1996). Both S. Lunn et al. (1999) and E. Roeder et al. (1996) discuss using alcohol as a co-solvent to decrease the DNAPL-phase density. However, Lunn used ethylene glycol to increase the alcohol-solution density to 1.11 g/mL, and a post-flush of polymer to maintain viscous stability, while Roeder used glycerin and sucrose solutions for a similar effect.
Despite positive results obtained from laboratory studies, the mobilization processes described above require aggressive chemical treatments and idealized flow conditions. The microemulsion process described by K. Kostarelos et al. (1998) typically reduces the potential for solubilization, and is relatively incapable of converting the density of DNAPLs to values lower than the groundwater density. Miscible alcohol flooding requires injecting large volumes of concentrated fluids. For example, S. Lunn et al.(1999b) injected a volume of 90% 1-propanol that was between six times and 61 times the total volume of the contaminant. Similarly, techniques to modify the aqueous-phase density require highly concentrated solutions (e.g., 60% NaI, or 65% ethylene glycol with 35% 1-propanol) that may significantly impact groundwater quality or subsurface ecology. Furthermore, because these techniques employ miscible displacements, field treatments would probably require large injections to overcome the effects of heterogeneity and dilution by dispersive mixing. Finally, it should be noted that many of the above-mentioned techniques only induce a near neutral density change with respect to the aqueous phase, which still leaves the difficult problem of producing an upward-directed flow in the field. (It should also be noted that most of the studies discussed above employed upward flow in columns or two-dimensional cells.)
An unfilled need exists for an inexpensive process for removing dense substance contaminants, while minimizing the risks of inadvertent downward mobilization of the contaminants.
We have discovered a novel method for the remediation of dense contaminants through the in-situ conversion of the dense contaminants into light contaminants. This method is a precursor treatment for remediation. It uses injection techniques and controlled destabilizing techniques to upwardly mobilize dense contaminants classified as DNAPLs. In one embodiment, a treating agent (e.g., polyaphron solution) comprising a light organic-phase fluid (e.g., kerosene, xylene, octane, hexane, etc.) is injected into the Earth""s subsurface, targeted at a contaminant source zone. A post-flush agent comprising a polyvalent cation (e.g., Al3+, Fe3+, etc.) is injected to destabilize the polyaphron, which induces in-situ mixing between the light organic-phase fluid and the dense contaminant. The dense contaminant is converted to a light contaminant (i.e., a contaminant having a density less than the density of groundwater), which assists upward mobilization once it has begun. Unlike prior methods of DNAPL density modifications, the novel process does not require cosolvents or phase partitioning. Rather, it uses in-situ mixing of a light-organic phase with the contaminant. The process also removes the risk of downward mobilization, allowing the use of other known methods of remediation to remove the contaminants from the soil.